Tandem breastfeeding a toddler and new baby can be a challenge, but with a little planning, it can work out quite well.
During your pregnancy, your milk supply will decrease. This is a normal reaction to the hormone levels in your body that are supporting your pregnancy, and efforts to increase your supply during this time will probably not be successful. During the third trimester your milk will transition back to colostrum in preparation for your new baby. This will continue to be made until after the new baby is born. And no, it's not harmful to have your toddler feeding from your breasts at this time — and there will still be plenty for your baby.
Once your new baby is here, it's best to allow your newborn to nurse first to get the milk she needs, then your toddler can drain your breasts after your baby has had a full feed. By allowing your baby to nurse first, draining either one or both breasts as she'll need to for a full feeding, you'll ensure that she's getting all the milk she needs for proper growth. Then when your toddler further drains your breasts after the baby has fed, your body will respond by increasing your overall milk production to accommodate both of their needs.